In recent years, attention has been drawn to an OFDM (Orthogonal Frequency Division Multiplexing) transmission system high in resistance to multipath interference that is a main factor in deterioration of transmission performance on transmission paths. OFDM is such a system that multiplexes a large number of (a few tens to hundreds) digital modulated signals with frequencies orthogonal to each other at a signal interval.
One of communication methods using OFDM transmission is disclosed in “Coded OFDM System using Guard Interval Length Control” IEICE, General Conference, B-5-92, 2001.
In this communication method, a base station (described as an access point in the above document) receives a known signal (preamble) transmitted from a mobile apparatus (described as a mobile terminal in the above document) and using the known signal, generates a delay profile, while measuring Carrier to Noise ratio (C/N ratio). Using the delay profile and C/N ratio, the base station determines an optimal guard interval, the modulation level of M-ary modulation and coding rate of FEC (Forward Error Correction) to transmit over the downlink.
Since the communication method employs a TDMA/TDD (Time Division Multiple Access/Time Division Duplex) system as an access scheme, the reversibility is used of transmission paths over the uplink (communication terminal to base station) and downlink (base station to communication terminal).
The communication method employs OFDM over both the uplink and downlink. In OFDM, by increasing the duration of a symbol, effects of delayed versions are decreased. Accordingly, since the duration of a symbol is increased, time resolution is insufficient, and it is thereby impossible to measure a delay profile accurately. When a guard interval is determined based on a delay profile generated under such a condition that the time resolution is insufficient, there is a risk that an unnecessary time is also set as a guard interval and the spectral efficiency deteriorates.